In the automotive industry there is a movement away from employing a standard 12 volt electrical system to employ a much higher voltage electrical system. The application of conventional 14 volt nominal solenoid activated fuel injectors to automotive electrical systems of much higher voltage such as, for example 42 volts nominal, presents problems to the fuel injector design. Assuming that the typical size of the fuel injector cannot change, and that the fuel injector will most likely be driven by a saturated switch electronic control unit (ECU), major modifications in the coil design of existing injectors will be required. For example, if an existing 14 ohm fuel injector coil consists of 525 turns of #34.5 AWG copper wire, the corresponding coil for a 42 volt operation must be a 126 ohms consisting of 1575 turns of #38.5 AWG copper wire. These calculations follow from the requirement of providing the same number of ampere-turns for an acceptable magnetic force and of maintaining the power dissipation the same in the injector. The use of extremely fine gauge wire is not feasible in the rugged fuel injector environment. For example, by way of comparison, wire size #36 AWG is generally the same thickness as a human hair. Thus, it is expected that the cost of manufacturing these fuel injectors will be higher than that of the conventional fuel injectors due to increased coil failures during manufacturing and assembly.
Since it may not be economically feasible to modify the coil of a fuel injector to operate at the higher voltages, modification of the driver circuit is another approach in widening the voltage range of a fuel injector. Typical methods of driving solenoid activated fuel injectors include using either peak and hold or saturated switch drivers. The peak and hold drivers reduce power consumption but may exhibit problems when the supply voltage increases to the point where the current peaks before the air gap has closed sufficiently for the hold current to latch the armature of the fuel injector. Also, the peak and hold driver circuit is more complex which increases cost.
Saturated switch drivers generally cause high power dissipation in the injector at high supply voltages and also slow the closing time of the injector because of excess energy stored in the magnetic circuit.
Accordingly, there is a need to provide a driver circuit for a solenoid activated fuel injector which provides lower power consumption from the power supply, does not peak too early, and permits injector operation over a large voltage range.